I agree that it really varies depending on the person and the circumstances.
Two months before our second cruise, I injured my knee rather seriously during a skiing accident (spent a month on crutches, it was a bad scene

), so while on the ship, I couldn't do as much of the stair walking and basically had to use the elevator all the time. I gained 5 pounds on a 7 day Eastern.
On our last cruise (7 Day Western), I took the stairs everywhere, and I was the one who would offer to run back to the room if someone forgot something. I stayed the same weight, and ate like a pig.
One hint for taking the stairs, keep your foot absolutely flat on the stair. When you push up, you are actually exercising your leg muscles more, than if you walk more on the ball of your foot. I know it sounds rather abstract, but the next time you walk up the stairs, walk normally and then try walking keeping you feet flat and you'll notice a difference
Katdb